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<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
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    <title>Document</title>
</head>
<body>
    <div>
        <p>圆的半径：<input id="r" type="text"></p>
        <p>圆的周长：<input id="cir" type="text"></p>
        <p>圆的面积：<input id="area" type="text"></p>
        <script>
            var r= prompt('请输入圆的半径');
            r=parseFloat(r) && Number(r);
            if(!isNaN(r)){
                var cir =Math.PI*r*2;
                var area = Math.PI*r*r;
                document.getElementById('r').value =r;
                document.getElementById('cir').value =cir.toFixed(2);
                document.getElementById('area').value =area.toFixed(2);
            }else{
                alert('请输入正确的数字');
            }
            </script>
    </div>
  
</body>
</html>